Unit 8. MOSFET Differential Amplifier Stage

The MOSFET differential amplifier stage is a relatively high-gain stage with grounded dc inputs, which requires no bypass capacitor. The stage also has the important advantage of having two inputs, one inverting and one noninverting. This is essential in operational amplifiers where one or the other (or both) is used, depending on the application. The two inputs also provide for operating the stage in the differential mode, where the input is not referenced to ground. The differential amplifier stage discussed here is an all-resistor circuit with source and drain resistors. In a modern integrated circuit, these are replaced with transistor current sources. However, the principle of operation is the same, and the resistor version can be implemented in our laboratory project.

Various segments of the differential amplifier comprise amplifier stages from all three terminal configurations: the common source, the common gate, and the source follower (common drain). In this unit, a detailed theoretical analysis of all three stages is undertaken (based on Level 1 SPICE). The linear models used include, progressively, the output conductance, gds, and the body-effect transconductance, gmb.

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